THERMAL STRAIN IN LEAD THIN-FILMS .4. EFFECTS OF MULTIPLE CYCLING TO 4.2-K

Using scanning electron microscopy and X-ray diffraction, strain, surface morphology and grain rotation were investigated in lead thin films which were deposited onto oxidized silicon wafers at room temperature and then thermally cycled up to 400 times between room temperature and liquid helium temperature. The strains measured at 4.2 K for films cycled 400 times were observed to have the same dependence on film thickness as that observed the first time the films were cooled to 4.2 K. In the films greater than 0.2 microm thick many discolation slip lines and hillocks were observed inside grains and on grain boundaries respectively after multiple cycling. The densities of the dislocation lines and the hillocks increased with the number of cycles. It was observed that repeated cycling caused rotation of (111)-oriented grains such that the (111) planes of different grains became more nearly coplanar and parallel to the substrate surface. The amount of grain rotation increased with the logarithm of the accumulated amount of plastic strain induced by the thermal cycling. © 1979.